Baba was on loan with the Royal Blues last season until he suffered an injury while playing for Ghana at the Africa Cup of Nations in January, effectively cutting short his campaign.
With the Tamale-born expected back in action in August, Schalke hold hopes of promptly resigning the player, who has also attracted interest from Newcastle United and Everton.
"Our team doctor met Abdul in London," Heidel told Bild.
"His knee injury is more complex and [his recovery] might be prolonged.
"We will wait for another four weeks and then decide with Baba and his agent whether [another loan] makes sense."
Baba made 13 Bundesliga appearances involving 7 starts before being knocked out by the injury.
The 22-year-old joined Chelsea in August 2015, but was forced to look elsewhere for regular action after playing 15 league games in his first season at Stamford Bridge.